Extreme rainfall and flooding from supercell thunderstorms

Supercell thunderstorms, the storm systems responsible for most tornadoes, have often been dismissed as flood hazards. The role of supercell thunderstorms as flood agents is examined through analyses of storm systems that occurred in Texas (5-6 May 1995), Florida (26 March 1992), Nebraska (20-21 June 1996), and Pennsylvania (18-19 July 1996). Particular attention is given to the "Dallas Supercell," which resulted in 16 deaths from flash flooding and more than $1 billion in property damage during the evening of 5 May 1995. Rainfall analyses using Weather Surveillance Radar-1988 Doppier (WSR-88D) reflectivity observations and special mesonet rain gauge observations from Dallas, Texas, show that catastrophic flash flooding resulted from exceptional rainfall rates at 5-60-min timescales. The spatial structure of extreme rainfall was linked to supercell structure and motion. The "Orlando Supercell" produced extreme rainfall rates (greater than 300 mm h^-1) at 1-5-min time-scales over a dense rain gauge network. The Nebraska and Pennsylvania storm systems produced record flooding over larger spatial scales than the Texas and Florida storms, by virtue of organization and motion of multiple storms over the same region. For both the Nebraska and Pennsylvania storms, extreme rainfall and tornadoes occurred in tandem. Severe rainfall measurement problems arise for supercell thunderstorms, both from conventional gauge networks and weather radar. It is hypothesized that supercell storms play a significant role in the "climatology" of extreme rainfall rates (100-yr return interval and greater) at short time intervals (1-60 min) in much of the central and eastern United States.